Laminated body

ABSTRACT

A nonaqueous electrolyte secondary battery separator that is not easily curled is provided by a laminated body including a porous base material containing a polyolefin-based resin and a porous layer on at least one surface of the porous base material. The difference between the white index of a surface of the porous base material after being irradiated with ultraviolet light with an intensity of 255 W/m 2  for 75 hours and the white index of the surface of the porous base material before irradiation is not more than 2.5. The porous layer contains a polyvinylidene fluoride-based resin which contains crystal form α in an amount of not less than 36 mol % with respect to 100 mol % of a total amount of the crystal form α and crystal form β contained in the resin.

This Nonprovisional application claims priority under 35 U.S.C. §119 onPatent Application No. 2016-123053 filed in Japan on Jun. 21, 2016, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a laminated body, and morespecifically, to a laminated body usable as a separator for a nonaqueouselectrolyte secondary battery (hereinafter referred to as a “nonaqueouselectrolyte secondary battery separator”).

BACKGROUND ART

Nonaqueous electrolyte secondary batteries such as a lithium-ionsecondary battery have a high energy density, and are thus in wide useas batteries for devices such as a personal computer, a mobiletelephone, and a portable information terminal. Such nonaqueouselectrolyte secondary batteries have recently been developed ason-vehicle batteries.

In a nonaqueous electrolyte secondary battery, the electrodes expand andcontract repeatedly as the nonaqueous electrolyte secondary battery ischarged and discharged. The electrodes and the separator thus causestress on each other. This, for example, causes the electrode activematerials to fall off and consequently increases the internalresistance, unfortunately resulting in a degraded cycle characteristic.In view of that, there has been proposed a technique of coating thesurface of a separator with an adhesive material such as polyvinylidenefluoride for increased adhesiveness between the separator and electrodes(see Patent Literatures 1 and 2). Coating a separator with an adhesivematerial, however, has been causing the separator to curl visibly. Acurled separator cannot be handled easily during production, which mayunfortunately lead to problems during battery preparation such asdefective rolling and defective assembly.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent No. 5355823 (Publication Date:    Nov. 27, 2013)-   [Patent Literature 2] Japanese Patent Application Publication,    Tokukai, No. 2001-118558 (Publication Date: Apr. 27, 2001)

SUMMARY OF INVENTION Technical Problem

The present invention has been accomplished in view of the above issue.It is an object of the present invention to sufficiently prevent aseparator from curling.

Solution to Problem

In order to attain the above object, the inventor of the presentinvention has conducted diligent research and have thus discovered thata separator capable of sufficiently preventing itself from curling canbe produced from a laminated body including (i) a porous base materialcontaining a polyolefin resin as a main component and (ii) a porouslayer disposed on the porous base material which porous layer contains apolyvinylidene fluoride-based resin (hereinafter also referred to as“PVDF-based resin”), the polyvinylidene fluoride-based resin havingmoderately controlled crystal forms. The inventor has also discoveredthat there is a correlation between (i) the amount of difference betweenthe white index (hereinafter also referred to as “WI”) of the porousbase material before the porous base material is irradiated withultraviolet light under predetermined conditions and the white index ofthe porous base material after the porous base material is irradiatedwith ultraviolet light under the predetermined conditions and (ii) thecycle characteristic of a nonaqueous electrolyte secondary battery to beproduced. The inventor has then obtained the knowledge that the abovedifference amount being not more than a predetermined value makes itpossible to provide a nonaqueous electrolyte secondary battery having anexcellent cycle characteristic.

In order to attain the above object, a laminated body in accordance withan embodiment of the present invention includes: a porous base materialcontaining a polyolefin-based resin as a main component; and a porouslayer on at least one surface of the porous base material, the porouslayer containing a polyvinylidene fluoride-based resin, the porous basematerial having a value of ΔWI of not more than 2.5, the ΔWI beingdefined in Formula (1) below,

ΔWI=WI₁−WI₀   Formula (1)

where WI represents a white index defined in American Standard TestMethods E313, WI₀ represents a WI value of a surface of the porous basematerial which WI value is measured with use of a spectrocolorimeterbefore the porous base material is irradiated with ultraviolet lightwith an intensity of 255 W/m², and WI₁ represents a WI value of thesurface of the porous base material which WI value is measured with useof the spectrocolorimeter after the porous base material is irradiatedwith ultraviolet light with an intensity of 255 W/m² for 75 hours, thepolyvinylidene fluoride-based resin containing crystal form α in anamount of not less than 36 mol % with respect to 100% by mass of a totalamount of the crystal form α and crystal form β contained in thepolyvinylidene fluoride-based resin, where the amount of the crystalform α is calculated from an absorption intensity at around 765 cm⁻¹ inan IR spectrum of the porous layer, and an amount of the crystal form βis calculated from an absorption intensity at around 840 cm⁻¹ in the IRspectrum of the porous layer.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the polyvinylidenefluoride-based resin contains (i) a homopolymer of vinylidene fluorideand/or (ii) a copolymer of vinylidene fluoride and at least one monomerselected from the group consisting of hexafluoropropylene,tetrafluoroethylene, trifluoroethylene, trichloroethylene, and vinylfluoride.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the polyvinylidenefluoride-based resin has a weight-average molecular weight of not lessthan 200,000 and not more than 3,000,000.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the porous layer containsa filler.

The laminated body in accordance with an embodiment of the presentinvention may preferably be arranged such that the filler has avolume-average particle size of not less than 0.01 μm and not more than10 μm.

A member for a nonaqueous electrolyte secondary battery (hereinafterreferred to as a “nonaqueous electrolyte secondary battery member”) inaccordance with an embodiment of the present invention includes: acathode; the laminated body; and an anode, the cathode, the laminatedbody, and the anode being arranged in this order.

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention includes the above laminated body asa separator.

Advantageous Effects of Invention

An embodiment of the present invention can prevent curls.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating how resin is extruded androlled for production of a sheet made of the resin.

DESCRIPTION OF EMBODIMENTS

The following description will discuss an embodiment of the presentinvention in detail. Note that numerical expressions such as “A to B”herein mean “not less than A and not more than B”.

<Laminated Body>

A laminated body in accordance with the present embodiment includes: aporous base material containing a polyolefin-based resin as a maincomponent; and a porous layer on at least one surface of the porous basematerial, the porous layer containing a polyvinylidene fluoride-basedresin, (a) the porous base material having a value of ΔWI of not morethan 2.5, the ΔWI being defined in Formula (1) below,

ΔWI=WI₁−WI₀   Formula (1)

where WI represents a white index defined in American Standard TestMethods E313, WI₀ represents a WI value of a surface of the porous basematerial which WI value is measured with use of a spectrocolorimeterbefore the porous base material is irradiated with ultraviolet lightwith an intensity of 255 W/m², and WI₁ represents a WI value of thesurface of the porous base material which WI value is measured with useof the spectrocolorimeter after the porous base material is irradiatedwith ultraviolet light with an intensity of 255 W/m² for 75 hours, (b)the polyvinylidene fluoride-based resin containing crystal form α in anamount of not less than 36 mol % with respect to 100 mol % of a totalamount of the crystal form α and crystal form β contained in thepolyvinylidene fluoride-based resin, where the amount of the crystalform α is calculated from an absorption intensity at around 765 cm⁻¹ inan IR spectrum of the porous layer, and an amount of the crystal form βis calculated from an absorption intensity at around 840 cm⁻¹ in the IRspectrum of the porous layer.

(1) Porous Base Material

A porous base material for the present embodiment is a base material fora laminated body of the present embodiment, and contains apolyolefin-based resin as a main component. A porous base material forthe present embodiment is preferably a microporous film. Specifically,the porous base material preferably contains, as a main component, apolyolefin-based resin that contains pores connected to one another andthat allows a gas, a liquid, or the like to pass therethrough from onesurface to the other. The porous base material may include a singlelayer or a plurality of layers.

A porous base material containing a polyolefin-based resin as a maincomponent means that the polyolefin-based resin component accounts fornormally not less than 50% by volume, preferably not less than 90% byvolume, more preferably not less than 95% by volume, of the entireporous base material. The polyolefin-based resin of the porous basematerial preferably contains a high molecular weight component having aweight-average molecular weight of 5×10⁵ to 15×10⁶. In a case where thepolyolefin-based resin of the porous base material has a weight-averagemolecular weight of not less than 1,000,000, a nonaqueous electrolytesecondary battery separator including (i) the porous base material and(ii) a later-described porous layer preferably has a higher strength.

Examples of the polyolefin-based resin include a high molecular weighthomopolymer (for example, polyethylene, polypropylene, or polybutene) ora copolymer (for example, an ethylene-propylene copolymer) produced bypolymerizing, for example, ethylene, propylene, 1-butene,4-methyl-1-pentene, or 1-hexene. The porous base material includes alayer containing only one of these polyolefin-based resins and/or alayer containing two or more of these polyolefin-based resins. Thepolyolefin-based resin is, in particular, preferably a high molecularweight polyethylene-based resin containing ethylene as a main componentbecause such a polyethylene-based resin is capable of preventing(shutting down) a flow of an excessively large electric current at alower temperature. The porous base material may contain a component(s)other than a polyolefin-based resin as long as such a component does notimpair the function of the layer.

Examples of the polyethylene-based resin include low-densitypolyethylene, high-density polyethylene, linear polyethylene(ethylene-α-olefin copolymer), and ultra-high molecular weightpolyethylene having a weight-average molecular weight of not less than1,000,000. Among these examples, ultra-high molecular weightpolyethylene having a weight-average molecular weight of not less than1,000,000 is further preferable.

The porous base material has a film thickness of preferably 4 μm to 40μm, more preferably 5 μm to 30 μm, still more preferably 6 μm to 15 μm.

The porous base material only needs to have a weight per unit area whichweight is determined as appropriate in view of the strength, filmthickness, weight, and handleability of the separator. Note, however,that the porous base material has a weight per unit area of preferably 4g/m² to 20 g/m², more preferably 4 g/m² to 12 g/m², still morepreferably 5 g/m² to 10 g/m², so as to allow a nonaqueous electrolytesecondary battery that includes a laminated body including the porousbase material to have a higher weight energy density and a higher volumeenergy density.

The porous base material has an air permeability of preferably 30sec/100 mL to 500 sec/100 mL, more preferably 50 sec/100 mL to 300sec/100 mL, in terms of Gurley values. A porous base material having anair permeability within the above range can have sufficient ionpermeability.

The porous base material has a porosity of preferably 20% by volume to80% by volume, more preferably 30% by volume to 75% by volume, so as to(i) retain a larger amount of electrolyte and (ii) obtain the functionof reliably preventing (shutting down) a flow of an excessively largeelectric current at a lower temperature. Further, in order to obtainsufficient ion permeability and prevent particles from entering thecathode and/or the anode, the porous base material has pores each havinga pore size of preferably not larger than 0.3 μm, more preferably notlarger than 0.14 μm.

A laminated body in accordance with the present embodiment includes aporous base material having a ΔWI (defined in Formula (1) above) valueof not more than 2.5. WI is an indicator of a color tone (whiteness) ofa sample, and is used to indicate, for example, (i) the fadingcharacteristic of a dye or (ii) the degree of oxidation degradation intransparent or white resin being processed. A higher WI value shouldindicate a higher degree of whiteness.

A porous base material can be produced by, for example, (i) a method ofadding a pore forming agent such as a filler or plasticizing agent to aresin such as polyolefin, shaping the resin into a sheet, then removingthe pore forming agent with use of an appropriate solvent, andstretching the sheet from which the pore forming agent has been removed,or (ii) a method of adding a pore forming agent to a resin such aspolyolefin, shaping the resin into a sheet, then stretching the sheet,and removing the pore forming agent from the stretched sheet.

The sheet can be produced by, for example, (i) extruding the resin(which contains a pore forming agent) from a T-die or the like and (ii)rolling the extruded resin with use of a pair of rolls into a thin film.FIG. 1 is a diagram schematically illustrating how resin is extruded androlled for production of a sheet made of the resin. FIG. 1 shows resin 1as a raw material for a separator, a T-die 2, rolls 3, and the distance4 between the T-die 2 and the rolls 3.

In the later-described Comparative Production Examples 1 and 2, forexample, the resin 1 had a temperature of 253° C. (ComparativeProduction Example 1) or 252° C. (Comparative Production Example 2)immediately before being extruded from the T-die 2, and the rolls 3 eachhad a surface temperature of 150° C. for the sheet production. Theseconditions are publicly known conditions that are normally adopted forseparator production. During the above operation, the resin 1 has a hightemperature while it is exposed to air from the time point at which itis extruded from the T-die 2 to the time point at which it comes intocontact with the rolls 3. The resin 1 thus comes into contact withoxygen, generating an oxide of the resin 1 as a result. This oxide willcause a side reaction when the battery is charged and discharged, andwill consequently decrease the life of the battery. The porous basematerial thus preferably contains such an oxide in as small an amount aspossible.

The above oxide is faded on ultraviolet irradiation. This should meanthat a porous base material contains a larger amount of the oxide in acase where there is a larger difference between the WI that the surfaceof the porous base material has before ultraviolet irradiation and theWI that the surface of the porous base material has after ultravioletirradiation.

In view of the above, the inventor of the present invention conductedresearch on the basis of the idea that producing a laminated bodyincluding a porous base material having a small WI difference (that is,a porous base material in which oxide is contained in only a smallamount) should (i) eliminate the influence of the oxide on the cyclecharacteristic of a battery to be produced and thus (ii) increase thelife of the battery. The research has revealed that an excellent cyclecharacteristic is exhibited by a nonaqueous electrolyte secondarybattery including, as a nonaqueous electrolyte secondary batteryseparator (hereinafter referred to also as “separator”), a laminatedbody including a porous base material having a ΔWI (defined in Formula(1) above) value of not more than 2.5.

As defined in Formula (1), ΔWI is the difference between (i) that WIvalue (WI₀) of a surface of a porous base material which is measuredwith use of a spectrocolorimeter before the porous base material isirradiated with ultraviolet light having an intensity of 255 W/m² and(ii) that WI value (WI₁) of the surface of the porous base materialwhich is measured with use of a spectrocolorimeter after the porous basematerial is irradiated with ultraviolet light with an intensity of 255W/m² for 75 hours. WI₀ is that WI value of the surface of the porousbase material which is a value before irradiation of 255 W/m²ultraviolet light (that is, before the start of irradiation of 255 W/m²ultraviolet light).

The spectrocolorimeter is, for example, suitably a integrating-spherespectrocolorimeter, which allows for easy and accurate WI measurement.An integrating-sphere spectrocolorimeter is a device for carrying outoptical spectrometric measurement by (i) irradiating a sample with lightof a xenon lamp and (ii) causing an integrating sphere that covers thevicinity of an irradiated portion of the sample to collect, in a lightreceiving section, light reflected from the sample. Anintegrating-sphere spectrocolorimeter allows for measurement of variousoptical parameters. Note that the spectrocolorimeter is not limited toan integrating-sphere spectrocolorimeter, and may be anyspectrocolorimeter that is capable of WI measurement.

The above “surface of a porous base material” refers to that portion ofthe porous base material which receives light emitted by thespectrocolorimeter. The spectrocolorimeter may be used for measurementof the WI value of the surface of the porous base material as explainedin the manual of the spectrocolorimeter to be used. The method for themeasurement is thus not limited to any particular one. However, in orderfor the light receiving section of the spectrocolorimeter to easilycollect light reflected by the porous base material, the porous basematerial is, for example, preferably placed on black paper before beingirradiated with light.

The porous base material is preferably irradiated with 255 W/m²ultraviolet light with use of a device capable of continuous ultravioletirradiation. The device can be, for example, a light resistance testerdefined in JIS B 7753 or a weather resistance tester (for example,Sunshine Weather Meter S80 available from Suga Tester). The ultravioletirradiation is carried out on a test piece for 75 hours at a relativehumidity of 50% with use of (i) a sunshine carbon arc light source (withfour pairs of ultra long-life carbon rods) set to have a dischargevoltage of 50 V and a discharge current of 60 A and (ii) a black panelhaving a temperature of 60° C.

The above tester is configured such that a metal plate to which a samplehas been attached is rotated around an ultraviolet lamp as the center sothat the sample is continuously exposed to ultraviolet light. A weatherresistance tester is capable of (i) irradiation involving use of a lightsource for emitting artificial light close to sunlight and (ii)intermittent water injection or (i) repetition of irradiation involvinguse of a light source for emitting artificial light close to sunlightand non-irradiation for darkness and (ii) spray of cold water onto theback surface of the test piece while it is dark. This makes it possibleto simulate conditions for a rainy weather (high humidity). For thepresent embodiment, however, it is only necessary to cause the porousbase material to be faded to a degree that allows for calculation of thedifference between WI₀ and WI₁. Thus, it is not necessary to simulateconditions for a rainy weather (high humidity).

In a case where ΔWI defined in Formula (1) has a value of not more than2.5, a nonaqueous electrolyte secondary battery including the porousbase material has a high discharge capacity maintaining ratio of morethan 70.0% even after 180 cycles of charge and discharge as demonstratedin Production Examples described later. The ΔWI value correlates to theamount of oxide contained in the porous base material as describedabove. Since a higher ΔWI value indicates a larger amount of oxidecontained, the ΔWI value is preferably as small as possible. The ΔWIvalue is essentially not more than 2.5, more preferably not more than2.3, further preferably not more than 2.2. The ΔWI value has a lowerlimit value of preferably not less than −10, more preferably not lessthan −5, most preferably 0.

A porous base material having a ΔWI value of not more than 2.5 can beproduced by reducing the time period during which the resin is exposedto air while it has a high temperature in the process of producing asheet. Referring to FIG. 1 as an example, the reduction can be achievedby, for example, (1) lowering the temperature of the resin 1 extrudedfrom the T-die 2 (extrusion temperature), (2) increasing the rate ofextruding the resin 1 to reduce the time period during which the resin 1is in contact with oxygen, or (3) reducing the distance 4 between theT-die 2 and the rolls 3.

In view of the balance between (i) the resin 1 needing to have a hightemperature for extrusion and (ii) reduction of the time period duringwhich the resin 1 is exposed to air while it has a high temperature, theextrusion temperature in (1) above is preferably 200° C. to 250° C.,more preferably 220° C. to 249° C., further preferably 240° C. to 248°C. The extrusion temperature refers to the temperature that the resinhas immediately before the resin is extruded from the discharge opening(for example, a T-die) of an extruder, and equals the temperature atwhich the discharge opening is set.

The rate of extrusion in (2) above cannot be generalized as it alsodepends on the processing capability of the extruder, but is preferably1 m/min to 10 m/min, more preferably 2 m/min to 8 m/min, furtherpreferably 2.5 m/min to 5 m/min.

As in Comparative Example 1 described later, an extrusion temperature ofhigher than 250° C. tended to result in an increased amount of oxidecontained in resin even with an increased rate of extrusion. Thus,setting the extrusion temperature within the above preferable range andalso increasing the rate of extrusion should further reduce the amountof oxide in the porous base material.

The distance 4 in (3) above is preferably as small as possible as longas it does not prevent the operation of the rolls 3. The rolls 3 eachhave a surface temperature of preferably 120° C. to 160° C., morepreferably 130° C. to 155° C., further preferably 140° C. to 150° C.

The above conditions are those that apply to the resin extrusion.Another effective method is (4) adding a surfactant to a solvent throughwhich a sheet produced with use of the rolls 3 is to be passed forremoval of the pore forming agent from the sheet. In a case where, forinstance, the solvent is an aqueous hydrochloric acid solution, and apore forming agent such as calcium carbonate dispersed in a sheet is tobe dissolved in the aqueous solution for removal, adding a surfactant tothe aqueous solution can increase the degree of permeation of theaqueous hydrochloric acid solution through the polyolefin-based resin ofthe sheet. This in turn allows oxide in the resin (which oxide, being anorganic substance, is normally insoluble in an aqueous hydrochloric acidsolution) to be dissolved more readily in the aqueous hydrochloric acidsolution, thereby promoting extraction of the oxide into the aqueoushydrochloric acid solution.

The surfactant may be an anionic surfactant, a cationic surfactant, anonionic surfactant, or an amphoteric surfactant. The surfactant is,however, preferably a nonionic surfactant as it is not easily affectedby an acid or alkali. Adding the surfactant in a larger amount allowsthe pore forming agent to be washed away (removed) and the oxide to beremoved with higher efficiency. Note, however, that adding thesurfactant in too large an amount may cause the surfactant to remain inthe separator. Thus, the surfactant is added in an amount of preferablynot less than 0.1% by weight and not more than 15% by weight, morepreferably 0.1% by weight to 10% by weight, with respect to 100% byweight of the cleaning liquid.

In a case where the solvent is water, for example, the temperature ofthe solvent (cleaning temperature) is not lower than 25° C. and nothigher than 60° C., more preferably not lower than 30° C. and not higherthan 55° C., particularly preferably not lower than 35° C. and nothigher than 50° C. This is because although a higher cleaningtemperature allows the pore forming agent to be removed with higherefficiency, too high a cleaning temperature will cause the cleaningliquid to evaporate. Note that the term “cleaning temperature” refers tothe temperature of the cleaning liquid with which the sheet has beenimmersed.

The sheet having been cleaned with the solvent may further be cleanedwith water. The cleaning with water is carried out at a water-cleaningtemperature of preferably not lower than 25° C. and not higher than 60°C., more preferably not lower than 30° C. and not higher than 55° C.,particularly preferably not lower than 35° C. and not higher than 50° C.This is because although a higher water-cleaning temperature allows thecleaning with water to be carried out with higher efficiency, too high awater-cleaning temperature will cause a cleaning liquid (water) toevaporate. Note that the term “water-cleaning temperature” refers to thetemperature of the water with which the sheet has been immersed.

Using one or more methods selected from (1) to (4) above in producing aporous base material can reduce the content of oxide in the porous basematerial, and thus allows for production of a porous base materialhaving a ΔWI value of not more than 2.5. In particular, it is morepreferable to combine the adjustment of the extrusion temperature forthe resin ((1) above) and the addition of a surfactant to the solvent((4) above) as such a combination makes it possible to easily adjustconditions and effectively remove oxide.

Examples of the filler (pore forming agent) include, but are notparticularly limited to, (i) an inorganic filler that can be dissolvedin a water-based solvent containing an acid, (ii) an inorganic fillerthat can be dissolved in a water-based solvent containing an alkali, and(iii) an inorganic filler that can be dissolved in a water-based solventconstituted mainly by water.

Examples of the inorganic filler that can be dissolved in a water-basedsolvent containing an acid include calcium carbonate, magnesiumcarbonate, barium carbonate, zinc oxide, calcium oxide, aluminumhydroxide, magnesium hydroxide, calcium hydroxide, and calcium sulfate.Among these, calcium carbonate is preferable because an inexpensive,fine powder of calcium carbonate can be obtained easily.

Examples of the inorganic filler that can be dissolved in a water-basedsolvent containing an alkali include silicic acid and zinc oxide. Amongthese, silicic acid is preferable because an inexpensive, fine powder ofsilicic acid can be obtained easily.

Examples of the inorganic filler that can be dissolved in a water-basedsolvent constituted mainly by water include calcium chloride, sodiumchloride, and magnesium sulfate.

Examples of the plasticizing agent (pore forming agent) include, but arenot particularly limited to, a low molecular weight hydrocarbon such asliquid paraffin.

A laminated body in accordance with the present embodiment includes aporous base material having a ΔWI value of not more than 2.5, whichmeans that the porous base material contains a smaller amount of oxidethan conventionally publicly known separators. Thus, the use of alaminated body in accordance with the present embodiment as a nonaqueouselectrolyte secondary battery separator can reduce side reactions causedduring charging and discharging of the nonaqueous electrolyte secondarybattery. This in turn makes it possible to provide a nonaqueouselectrolyte secondary battery that exhibits an excellent cyclecharacteristic.

<Laminated Body>

Further, a laminated body of the present embodiment includes a publiclyknown porous layer(s) such as an adhesive layer, a heat-resistant layer,and a protective layer.

The porous base material is more preferably subjected to ahydrophilization treatment before a porous layer is formed, that is,before the porous base material is coated with a coating solution(described later). Performing a hydrophilization treatment on the porousbase material further improves coating easiness of the coating solutionand thus allows a more uniform porous layer to be formed. Thehydrophilization treatment is effective in a case where water accountsfor a high proportion of a solvent (dispersion medium) contained in thecoating solution.

Specific examples of the hydrophilization treatment include publiclyknown treatments such as (i) a chemical treatment involving an acid, analkali, or the like, (ii) a corona treatment, and (iii) a plasmatreatment. Among these hydrophilization treatments, the corona treatmentis more preferable because the corona treatment makes it possible to notonly hydrophilize the porous base material within a relatively shorttime period, but also hydrophilize only a surface and its vicinity ofthe porous base material to leave the inside of the porous base materialunchanged in quality.

(Porous Layer)

The porous layer is preferably a resin layer containing a resin. Theresin in the porous layer is preferably (i) insoluble in the electrolyteof a nonaqueous electrolyte secondary battery to be produced and (ii)electrochemically stable when the nonaqueous electrolyte secondarybattery is in normal use. In a case where the porous layer is disposedon one surface of the porous base material, the porous layer is disposedpreferably on a surface of the porous base material which surface facesthe cathode of the nonaqueous electrolyte secondary battery, morepreferably on a surface of the porous base material which surface is incontact with the cathode.

The porous layer for the present embodiment contains a polyvinylidenefluoride-based resin, the polyvinylidene fluoride-based resin containingcrystal form α in an amount of not less than 36 mol % with respect to100 mol % of the total amount of the crystal form α and crystal form βcontained in the polyvinylidene fluoride-based resin.

The amount of the crystal form α is calculated from an absorptionintensity at around 765 cm⁻¹ in the IR spectrum of the porous layer, andthe amount of the crystal form β is calculated from an absorptionintensity at around 840 cm⁻¹ in the IR spectrum of the porous layer.

The porous layer for the present embodiment contains a polyvinylidenefluoride-based resin (PVDF-based resin). The porous layer contains alarge number of pores connected to one another, and thus allows a gas ora liquid to pass therethrough from one surface to the other. Further, ina case where the porous layer for the present embodiment is used as aconstituent member of a nonaqueous electrolyte secondary batteryseparator, the porous layer can be a layer capable of adhering to anelectrode as the outermost layer of the separator.

Examples of the PVDF-based resin include homopolymers of vinylidenefluoride (that is, polyvinylidene fluoride); copolymers (for example,polyvinylidene fluoride copolymer) of vinylidene fluoride and othermonomer(s) polymerizable with vinylidene fluoride; and mixtures of theabove polymers. Examples of the monomer polymerizable with vinylidenefluoride include hexafluoropropylene, tetrafluoroethylene,trifluoroethylene, trichloroethylene, and vinyl fluoride. The presentembodiment can use (i) one kind of monomer or (ii) two or more kinds ofmonomers selected from above. The PVDF-based resin can be synthesizedthrough emulsion polymerization or suspension polymerization.

The PVDF-based resin contains vinylidene fluoride at a proportion ofnormally not less than 85 mol %, preferably not less than 90 mol %, morepreferably not less than 95 mol %, further preferably not less than 98mol %. A PVDF-based resin containing vinylidene fluoride at a proportionof not less than 85 mol % is more likely to allow a porous layer to havea mechanical strength against pressure and a heat resistance againstheat during battery production.

The porous layer can also preferably contain two kinds of PVDF-basedresins (that is, a first resin and a second resin below) that differfrom each other in terms of, for example, the hexafluoropropylenecontent.

The first resin is (i) a vinylidene fluoride-hexafluoropropylenecopolymer containing hexafluoropropylene at a proportion of more than 0mol % and not more than 1.5 mol % or (ii) a vinylidene fluoridehomopolymer (containing hexafluoropropylene at a proportion of 0 mol %).

The second resin is a vinylidene fluoride-hexafluoropropylene copolymercontaining hexafluoropropylene at a proportion of more than 1.5 mol %.

A porous layer containing the two kinds of PVDF-based resins adheresbetter to an electrode than a porous layer not containing one of the twokinds of PVDF-based resins. Further, a porous layer containing the twokinds of PVDF-based resins adheres better to another layer (for example,the porous base material layer) included in a nonaqueous electrolytesecondary battery separator, with the result of a higher peel forcebetween the two layers, than a porous layer not containing one of thetwo kinds of PVDF-based resins. The first resin and the second resin arepreferably mixed at a mixing ratio (mass ratio, first resin:secondresin) of 15:85 to 85:15.

The PVDF-based resin has a weight-average molecular weight of preferably200,000 to 3,000,000. A PVDF-based resin having a weight-averagemolecular weight of not less than 200,000 tends to allow a porous layerto attain a mechanical property enough for the porous layer to endure aprocess of adhering the porous layer to an electrode, thereby allowingthe porous layer and the electrode to adhere to each other sufficiently.A PVDF-based resin having a weight-average molecular weight of not morethan 3,000,000 tends to not cause the coating solution, which is to beapplied to form a porous layer, to have too high a viscosity, whichallows the coating solution to have excellent shaping easiness. Theweight-average molecular weight of the PVDF-based resin is morepreferably 200,000 to 2,000,000, further preferably 500,000 to1,500,000.

The PVDF-based resin has a fibril diameter of preferably 10 nm to 1000nm in view of the cycle characteristic of a nonaqueous electrolytesecondary battery containing the porous layer.

The porous layer for the present embodiment may contain a resin otherthan the PVDF-based resin. Examples of the other resin includestyrene-butadiene copolymers; homopolymers or copolymers of vinylnitriles such as acrylonitrile and methacrylonitrile; and polyetherssuch as polyethylene oxide and polypropylene oxide.

The porous layer for the present embodiment may contain a filler. Thefiller may be an inorganic or organic filler. In a case where the porouslayer for the present embodiment contains a filler, the filler iscontained at a proportion of preferably not less than 1% by mass and notmore than 99% by mass, more preferably not less than 10% by mass and notmore than 98% by mass, with respect to the total amount of thepolyvinylidene fluoride-based resin and the filler combined. Containinga filler allows a separator including the porous layer to have improvedslidability and heat resistance, for example. The filler may be anyinorganic or organic filler that is stable in a nonaqueous electrolyteand that is stable electrochemically. The filler preferably has aheat-resistant temperature of not lower than 150° C. to ensure safety ofthe battery.

Examples of the organic filler include: crosslinked polymethacrylic acidesters such as crosslinked polyacrylic acid, crosslinked polyacrylicacid ester, crosslinked polymethacrylic acid, and crosslinked polymethylmethacrylate; fine particles of crosslinked polymers such as crosslinkedpolysilicone, crosslinked polystyrene, crosslinked polydivinyl benzene,a crosslinked product of a styrene-divinylbenzene copolymer, polyimide,a melamine resin, a phenol resin, and a benzoguanamine-formaldehydecondensate; and fine particles of heat-resistant polymers such aspolysulfone, polyacrylonitrile, polyaramid, polyacetal, andthermoplastic polyimide.

A resin (polymer) contained in the organic filler may be a mixture, amodified product, a derivative, a copolymer (a random copolymer, analternating copolymer, a block copolymer, or a graft copolymer), or acrosslinked product of any of the molecular species listed above asexamples.

Examples of the inorganic filler include metal hydroxides such asaluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromiumhydroxide, zirconium hydroxide, nickel hydroxide, and boron hydroxide;metal oxides such as alumina and zirconia, and hydrates thereof;carbonates such as calcium carbonate and magnesium carbonate; sulfatessuch as barium sulfate and calcium sulfate; and clay minerals such ascalcium silicate and talc. The inorganic filler is preferably a metalhydroxide, a hydrate of a metal oxide, or a carbonate to improve thesafety of the battery, for example, to impart fire retardance. Theinorganic filler is preferably a metal oxide in terms of insulation andoxidation resistance.

The present embodiment may use (i) only one filler or (ii) two or morekinds of fillers in combination. Alternatively, the organic filler(s)and the inorganic filler(s) may be used in combination.

The filler has a volume average particle size of preferably 0.01 μm to10 μm in order to ensure (i) fine adhesion and fine slidability and (ii)shaping easiness of the laminated body. The volume average particle sizehas a lower limit of more preferably not less than 0.05 μm, furtherpreferably not less than 0.1 μm. The volume average particle size has anupper limit of more preferably not more than 5 μm, further preferablynot more than 1 μm.

The filler may have any shape. The filler may, for example, be aparticulate filler. Example shapes of the particles include a sphere, anellipse, a plate shape, a bar shape, and an irregular shape. In order toprevent a short circuit in the battery, the particles are preferably (i)plate-shaped particles or (ii) primary particles that are notaggregated.

The filler forms fine bumps on a surface of the porous layer, therebyimproving the slidability. Thus, a filler including (i) plate-shapedparticles or (ii) primary particles that are not aggregated forms finerbumps on a surface of the porous layer so that the porous layer adheresbetter to an electrode.

The porous layer for the present embodiment has an average thickness ofpreferably 0.5 μm to 10 μm, more preferably 1 μm to 5 μm, on one surfaceof the porous base material in order to ensure adhesion to an electrodeand a high energy density.

If the porous layer has a thickness of less than 0.5 μm on one surfaceof the porous base material, it will be impossible to, in a case wherethe laminated body is included in a nonaqueous electrolyte secondarybattery, sufficiently prevent an internal short circuit caused by, forexample, breakage of the nonaqueous electrolyte secondary battery.Further, such a porous layer can retain only a smaller amount ofelectrolyte.

If the porous layer has a thickness of more than 10 μm on one surface ofthe porous base material, the laminated body in a nonaqueous electrolytesecondary battery will have an increased resistance to permeation oflithium ions over the entire region of the laminated body. Thus,repeating charge-and-discharge cycles will degrade the cathode of thenonaqueous electrolyte secondary battery, with the result of a degradedrate characteristic and a degraded cycle characteristic. Further, such aporous layer will increase the distance between the cathode and theanode, with the result of a larger nonaqueous electrolyte secondarybattery.

In a case where the porous layer is disposed on both surfaces of theporous base material, the physical properties of the porous layer thatare described below at least refer to the physical properties of theporous layer disposed on a surface of the porous base material whichsurface faces the cathode of the nonaqueous electrolyte secondarybattery including the laminated body.

The porous layer only needs to have a weight per unit area (per onesurface thereof) which weight is appropriately determined in view of thestrength, film thickness, weight, and handleability of the laminatedbody. In a case where the laminated body is included in a nonaqueouselectrolyte secondary battery, the porous layer normally has a weightper unit area of preferably 0.5 g/m² to 20 g/m², more preferably 0.5g/m² to 10 g/m².

The porous layer which has a weight per unit area which weight fallswithin the above numerical range allows a nonaqueous electrolytesecondary battery including the porous layer to have a higher weightenergy density and a higher volume energy density. If the weight perunit area of the porous layer is beyond the above range, a nonaqueouselectrolyte secondary battery including the laminated separator will beheavy.

The porous layer has a porosity of preferably 20% by volume to 90% byvolume, more preferably 30% by volume to 80% by volume, in order toachieve sufficient ion permeability. The pore diameter of the pores inthe porous layer is preferably not more than 1.0 μm, more preferably notmore than 0.5 μm. In a case where the pores each have such a porediameter, a nonaqueous electrolyte secondary battery that includes alaminated body including the porous layer can achieve sufficient ionpermeability.

The laminated body has an air permeability of preferably 30 sec/100 mLto 1000 sec/100 mL, more preferably 50 sec/100 mL to 800 sec/100 mL interms of Gurley values. A laminated body having such an air permeabilityachieves sufficient ion permeability in a case where the laminated bodyis used as a member of a nonaqueous electrolyte secondary battery.

An air permeability larger than the above range means that the laminatedbody has a high porosity and thus has a coarse laminated structure. Thismay result in a separator having a lower strength and thus having aninsufficient shape stability at high temperatures in particular. An airpermeability smaller than the above range may, on the other hand,prevent the laminated body from having sufficient ion permeability whenused as a member of a nonaqueous electrolyte secondary battery and thusdegrade the battery characteristics of the nonaqueous electrolytesecondary battery.

A laminated body in accordance with the present embodiment includes aporous base material having a ΔWI value of not more than 2.5 asdescribed above, which means that the porous base material contains asmaller amount of oxide than conventionally publicly known separators.Thus, the laminated body can, similarly to the porous base material,reduce side reactions caused during charging and discharging of thenonaqueous electrolyte secondary battery. This in turn makes it possibleto provide a nonaqueous electrolyte secondary battery that exhibits anexcellent cycle characteristic.

<Crystal Forms of PVDF-Based Resin>

The PVDF-based resin in the porous layer for use in the presentembodiment contains crystal form α in an amount of not less than 36 mol%, preferably not less than 63 mol %, more preferably not less than 70mol %, further preferably not less than 80 mol %, with respect to 100mol % of the total amount of crystal form α and crystal form βcontained. Further, the amount of crystal form α is preferably not morethan 95 mol %. Containing crystal form α in an amount of not less than36 mol % allows a laminated body including the porous layer to be usedas a member of a nonaqueous electrolyte secondary battery such as anonaqueous electrolyte secondary battery separator that is not easilycurled.

A porous layer for use in the present embodiment is usable as a memberof a nonaqueous electrolyte secondary battery which member is not easilycurled for the following reason.

A laminated body of the present embodiment can prevent itself fromcurling presumably because, for example, (a) a smaller content of thePVDF-based resin having crystal form β, which PVDF-based resin stronglyadheres to the porous base material, allows the porous layer to bedeformed to only a moderately smaller degree in response to deformationof the porous base material and/or (b) a larger content of thePVDF-based resin having crystal form α, which PVDF-based resin is highin rigidity, allows the porous layer to be more resistant todeformation.

The PVDF-based resin having crystal form α is arranged such that in thePVDF skeleton contained in the polymer of the PVDF-based resin, withrespect to a fluorine atom (or a hydrogen atom) bonded to a singlemain-chain carbon atom in the molecular chains contained in the PVDFskeleton, one adjacent carbon atom is bonded to a hydrogen atom (or afluorine atom) having a trans position relative to the above fluorineatom (or the above hydrogen atom), and the other (opposite) adjacentcarbon atom is bonded to a hydrogen atom (or a fluorine atom) having agauche position (positioned at an angle of 60°) relative to the abovefluorine atom (or the above hydrogen atom), wherein two or more suchconformations are chained consecutively as follows:

(TGTG structure)   [Math. 1]

and the molecular chains each have the following type:

TGTG  [Math. 2]

wherein the respective dipole moments of C—F₂ and C—H₂ bonds each have acomponent perpendicular to the molecular chain and a component parallelto the molecular chain.

The PVDF-based resin having crystal form α has characteristic peaks(characteristic absorptions) at around 1,212 cm⁻¹, around 1,183 cm⁻¹,and around 765 cm⁻¹ in its IR spectrum. The PVDF-based resin havingcrystal form α has characteristic peaks at around 2θ=17.7°, around2θ=18.3°, and around 2θ=19.9° in a powder X-ray diffraction analysis.

The PVDF-based resin having crystal form β is arranged such that in thePVDF skeleton contained in the polymer of the PVDF-based resin, amain-chain carbon atom in the molecular chains contained in the PVDFskeleton is adjacent to two carbon atoms bonded to a fluorine atom and ahydrogen atom, respectively, having a trans conformation (TT-typeconformation), that is, the fluorine atom and the hydrogen atom bondedrespectively to the two adjacent carbon atoms are positioned at an angleof 180° to the direction of the carbon-carbon bond.

The PVDF-based resin having crystal form β may be arranged such that thepolymer of the PVDF-based resin contains a PVDF skeleton that has aTT-type conformation in its entirety. The PVDF-based resin havingcrystal form β may alternatively be arranged such that a portion of thePVDF skeleton has a TT-type conformation and that the PVDF-based resinhaving crystal form β has a molecular chain of the TT-type conformationin at least four consecutive PVDF monomeric units. In either case, (i)the carbon-carbon bond, in which the TT-type conformation constitutes aTT-type main chain, has a planar zigzag structure, and (ii) therespective dipole moments of C—F₂ and C—H₂ bonds each have a componentperpendicular to the molecular chain.

The PVDF-based resin having crystal form β has characteristic peaks(characteristic absorptions) at around 1,274 cm⁻¹, around 1,163 cm⁻¹,and around 840 cm⁻¹ in its IR spectrum. The PVDF-based resin havingcrystal form β has a characteristic peak at around 2θ=21° in a powderX-ray diffraction analysis.

A PVDF-based resin having crystal form γ is arranged such that thepolymer of the PVDF-based resin contains a PVDF skeleton that has aconformation in which TT-type conformations and TG-type conformationsappear consecutively and alternately. The PVDF-based resin havingcrystal form γ has characteristic peaks (characteristic absorptions) ataround 1,235 cm⁻¹ and around 811 cm⁻¹ in its IR spectrum. The PVDF-basedresin having crystal form γ has a characteristic peak at around 2θ=18°in a powder X-ray diffraction analysis.

<Method for Calculating Content Rates of Crystal Form α and Crystal Formβ in PVDF-Based Resin>

The respective content rates of crystal form α and crystal form β in thePVDF-based resin can be calculated by, for example, the methods (i) to(iii) below.

(i) Calculation Formula

Law of Beer: A=εbC   (1)

where A represents an absorbance, ε represents a molar extinctioncoefficient, b represents an optical path length, and C represents aconcentration.

Assuming that on the basis of the above formula (1), A^(α) representsthe absorbance of the characteristic absorption of crystal form α, A^(β)represents the absorbance of the characteristic absorption of crystalform β, ε^(α) represents the molar extinction coefficient of thePVDF-based resin having crystal form α, ε^(β) represents the molarextinction coefficient of the PVDF-based resin having crystal form β,C^(α) represents the concentration of the PVDF-based resin havingcrystal form α, and C^(β) represents the concentration of the PVDF-basedresin having crystal form β, the respective proportions of therespective absorbances of crystal form α and crystal form β areexpressed as follows:

A ^(β) /A ^(α)=(ε^(β)/ε^(α))×(C ^(β) /C ^(α))  (1a)

Assuming that a correction factor (ε^(β)/ε^(α)) for the molar extinctioncoefficient is E^(β/α), the content rate F(β)=(C^(β)/(C^(α)+C^(β))) ofthe PVDF-based resin having crystal form β with respect to the crystalform α and crystal form β combined is expressed by the following formula(2a):

$\begin{matrix}\begin{matrix}{{F(\beta)} = {\left\{ {\left( {1/E^{\beta/\alpha}} \right) \times \left( {A^{\alpha}/A^{\beta}} \right)} \right\}/\left\{ {1 + {\left( {1/E^{\beta/\alpha}} \right) \times \left( {A^{\alpha}/A^{\beta}} \right)}} \right\}}} \\{= {A^{\beta}/\left\{ {\left( {E^{\beta/\alpha} \times A^{\alpha}} \right) + A^{\beta}} \right\}}}\end{matrix} & \left( {2a} \right)\end{matrix}$

Thus, in a case where the correction factor E^(β/α) is determined, thecontent rate F(β) of the PVDF-based resin having crystal form β withrespect to the crystal form α and crystal form β combined can becalculated from an actual measurement of the absorbance A^(α) of thecharacteristic absorption of crystal form α and an actual measurement ofthe absorbance A^(β) of the characteristic absorption of crystal form β.Further, the content rate F(α) of the PVDF-based resin having crystalform α with respect to the crystal form α and crystal form β combinedcan be calculated from F((β) calculated as above.

(ii) Method for Determining Correction Factor E^(β/α)

A sample of a PVDF-based resin having only crystal form α is mixed witha sample of a PVDF-based resin having only crystal form β forpreparation of a sample with a known content rate F((β) of thePVDF-based resin having crystal form β. The IR spectrum of the preparedsample is measured. Then, measurements are made of the absorbance (peakheight) A^(α) of the characteristic absorption of crystal form α and theabsorbance (peak height) A^(β) of the characteristic absorption ofcrystal form β in the IR spectrum measured above.

Subsequently, A^(α) and A^(β) are substituted into the formula (3a)below, into which the formula (2a) is solved for E^(β/α), to determine acorrection factor E^(β/α).

E ^(β/α) ={A ^(β)×(1−F(β))}/(A ^(α) ×F(β))   (3a)

Measurements are made of respective IR spectrums of a plurality ofsamples having respective mixing ratios different from each other. Therespective correction factors E^(β/α) of the plurality of samples aredetermined by the above method, and the average of the correctionfactors E^(β/α) is then calculated.

(iii) Calculation of Respective Content Rates of Crystal Form α andCrystal Form β in Sample

For each sample, the content rate F(α) of the PVDF-based resin havingcrystal form α with respect to the crystal form α and crystal form βcombined is calculated on the basis of the average correction factorE^(β/α) calculated in (ii) above and the result of measurement of the IRspectrum of the sample.

Specifically, the content rate F(α) is calculated as follows: Alaminated body including the above porous layer is prepared by apreparation method described later. A portion of the laminated body iscut out for preparation of a measurement sample. Then, the infraredabsorption spectrum of the measurement sample at wave numbers from 4000cm⁻¹ to 400 cm⁻¹ (measurement range) is measured at room temperature(approximately 25° C.) with use of an FT-IR spectrometer (available fromBruker Optics K.K. model: ALPHA Platinum-ATR) with a resolution of 4cm⁻¹ and 512 times of scanning. The measurement sample cut out ispreferably in the shape of an 80 mm×80 mm square. The size and shape ofthe measurement sample are, however, not limited to that; themeasurement sample simply needs to be so sized as to allow its infraredabsorption spectrum to be measured. Then, from the spectrum measured,the absorption intensity (A^(α)) at 765 cm⁻¹ (characteristic absorptionof crystal form α) and the absorption intensity (A^(β)) at 840 cm⁻¹(characteristic absorption of crystal form β) are determined. Thestarting point and end point of a waveform formed with the wave numberset as a peak are connected with a straight line, where the lengthbetween the straight line and the peak wave number (peak top) denote anabsorption intensity. For crystal form α, a maximum possible absorptionintensity within the wave number range of 775 cm⁻¹ to 745 cm⁻¹ isassumed to be the absorption intensity (A^(α)) at 765 cm⁻¹. For crystalform β, a maximum possible absorption intensity within the wave numberrange of 850 cm⁻¹ to 815 cm⁻¹ is assumed to be the absorption intensity(A^(β)) at 840 cm⁻¹. Note that the content rate F(α) of crystal form αherein is calculated on the assumption of the average correction factorE^(β/α) being 1.681 (with reference to Japanese Patent ApplicationPublication, Tokukai, No. 2005-200623). The calculation uses thefollowing formula (4a):

F(α)(%)=[1−{absorption intensity(A ^(β)) at 840 cm⁻¹/(absorptionintensity (A ^(α)) at 765 cm⁻¹×correction factor (E^(β/α))(1.681)+absorption intensity (A ^(β)) at 840 cm⁻¹)}]×100   (4a)

[Method for Producing Porous Layer and Method for Producing LaminatedBody]

A porous layer and laminated body for the present embodiment may each beproduced by any production method, and may each be produced by any ofvarious methods.

In an example method, a porous layer containing a PVDF-based resin andoptionally a filler is formed, through one of the processes (1) to (3)below, on a surface of a polyolefin-based resin microporous film to be aporous base material. In the case of the process (2) or (3), a porouslayer deposited is dried for removal of the solvent. In the processes(1) to (3), the coating solution, in the case of production of a porouslayer containing a filler, preferably contains a filler dispersedtherein and a PVDF-based resin dissolved therein.

The coating solution for use in a method for producing a porous layerfor the present embodiment can be prepared normally by (i) dissolving,in a solvent, a resin to be contained in the porous layer for thepresent embodiment and (ii) dispersing, in the solvent, fine particlesto be contained in the porous layer for the present embodiment.

(1) A process of (i) coating a surface of a porous base material with acoating solution containing fine particles of a PVDF-based resin to becontained in a porous layer and optionally fine particles of a fillerand (ii) drying the surface of the porous base material to remove thesolvent (dispersion medium) from the coating solution for formation of aporous layer.

(2) A process of (i) coating a surface of a porous base material with acoating solution containing fine particles of a PVDF-based resin to becontained in a porous layer and optionally fine particles of a fillerand then (ii) immersing the porous base material into a depositionsolvent (which is a poor solvent for the PVDF-based resin) fordeposition of a porous layer containing the PVDF-based resin andoptionally the filler.

(3) A process of (i) coating a surface of a porous base material with acoating solution containing fine particles of a PVDF-based resin to becontained in a porous layer and optionally fine particles of a fillerand then (ii) making the coating solution acidic with use of alow-boiling-point organic acid for deposition of a porous layercontaining the PVDF-based resin and optionally the filler.

The solvent (dispersion medium) in the coating solution may be anysolvent that does not adversely affect the porous base material, thatallows a PVDF-based resin to be dissolved or dispersed therein uniformlyand stably, and that allows a filler to be dispersed therein uniformlyand stably. Examples of the solvent (dispersion medium) includeN-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide,acetone, and water.

The deposition solvent can be, for example, another solvent (hereinafteralso referred to as “solvent X”) that is dissolvable in the solvent(dispersion medium) contained in the coating solution and that does notdissolve the PVDF-based resin contained in the coating solution. Thesolvent (dispersion medium) can be efficiently removed from the coatingsolution by (i) immersing, in the solvent X, a porous base material towhich the coating solution has been applied and on which a coating filmhas been formed, for replacement of the solvent (dispersion medium) inthe coating film on the porous base material or a support with thesolvent X and then (ii) evaporating the solvent X. The depositionsolvent is preferably isopropyl alcohol or t-butyl alcohol, for example.

For the process (3), the low-boiling-point organic acid can be, forexample, paratoluene sulfonic acid or acetic acid.

The coating solution may be prepared by any method that allows thecoating solution to satisfy conditions such as the resin solid content(resin concentration) and the fine-particle amount that are necessary toproduce a desired porous layer. Specific examples of the method forpreparing a coating solution include a mechanical stirring method, anultrasonic dispersion method, a high-pressure dispersion method, and amedia dispersion method. The fine particles may be dispersed in thesolvent (dispersion medium) with use of a conventionally publicly knowndispersing device such as a three-one motor, a homogenizer, a media-typedispersing device, or a pressure-type dispersing device. Further, thecoating solution may be prepared simultaneously with wet grinding offine particles by supplying into a wet grinding device a liquid in whicha resin is dissolved or swollen or an emulsified liquid of a resinduring wet grinding carried out to produce fine particles having adesired average particle diameter. In other words, the wet grinding offine particles and the preparation of a coating solution may be carriedout simultaneously in a single step. The coating solution may contain anadditive(s) such as a dispersing agent, a plasticizing agent, a surfaceactive agent, and a pH adjusting agent as a component(s) other than theresin and the fine particles as long as such an additive does notprevent an object of the present invention from being attained. Theadditive may be added in an amount that does not prevent an object ofthe present invention from being attained.

The coating solution may be applied to the porous base material by anymethod, that is, a porous layer may be formed by any method on a surfaceof a porous base material that may have been subjected to ahydrophilization treatment as necessary. In a case where a porous layeris disposed on each of both surfaces of the porous base material, (i) asequential deposition method may be used, in which a porous layer isformed on one surface of the porous base material, and another porouslayer is subsequently formed on the other surface of the porous basematerial, or (ii) a simultaneous deposition method may be used, in whichporous layers are formed simultaneously on both surfaces of the porousbase material. A porous layer can be formed (that is, a laminated bodycan be produced) by, for example, (i) a method of applying the coatingsolution directly to a surface of the porous base material and thenremoving the solvent (dispersion medium), (ii) a method of applying thecoating solution to an appropriate support, removing the solvent(dispersion medium) for formation of a porous layer, thenpressure-bonding the porous layer to the porous base material, andpeeling the support off, (iii) a method of applying the coating solutionto a surface of an appropriate support, then pressure-bonding the porousbase material to that surface, then peeling the support off, and thenremoving the solvent (dispersion medium), or (iv) a method of immersingthe porous base material into the coating solution for dip coating andthen removing the solvent (dispersion medium). The thickness of theporous layer can be controlled by adjusting, for example, the thicknessof the coating film in a wet state (wet) after the coating, the weightratio between the resin and the fine particles, and the solid contentconcentration (that is, the sum of the resin concentration and thefine-particle concentration) of the coating solution. The support canbe, for example, a resin film, a metal belt, or a drum.

The coating solution may be applied to the porous base material orsupport by any method that can achieve a necessary weight per unit areaand a necessary coating area. The coating solution can be applied by aconventionally publicly known method. Specific examples include agravure coater method, a small-diameter gravure coater method, a reverseroll coater method, a transfer roll coater method, a kiss coater method,a dip coater method, a knife coater method, an air doctor blade coatermethod, a blade coater method, a rod coater method, a squeeze coatermethod, a cast coater method, a bar coater method, a die coater method,a screen printing method, and a spray coating method.

The solvent (dispersion medium) is typically removed by a drying method.Examples of the drying method include natural drying, air-blow drying,heat drying, and drying under reduced pressure. The solvent (dispersionmedium) can, however, be removed by any method that allows the solvent(dispersion medium) to be removed sufficiently. The solvent (dispersionmedium) contained in the coating solution may be replaced with anothersolvent before a drying operation. The solvent (dispersion medium) canbe replaced with another solvent for removal by, for example, a methodof (i) preparing another solvent (hereinafter referred to as “solventX”) that dissolves the solvent (dispersion medium) contained in thecoating solution and that does not dissolve the resin contained in thecoating solution, (ii) immersing the porous base material or support, towhich the coating solution has been applied and on which a coating filmhas been formed, into the solvent X to replace the solvent (dispersemedium) in the coating film on the porous base material or support withthe solvent X, and (iii) evaporating the solvent X. This method allowsthe solvent (dispersion medium) to be removed efficiently from thecoating solution. In a case where the coating film, formed on the porousbase material or support by applying the coating solution thereto, isheated when removing the solvent (dispersion medium) or solvent X fromthe coating film, the coating film is desirably heated at a temperaturethat does not decrease the air permeability of the porous base material,specifically within a range of 10° C. to 120° C., preferably within arange of 20° C. to 80° C., to prevent pores in the porous base materialfrom contracting to decrease the air permeability of the porous basematerial.

The solvent (dispersion medium) is preferably removed by, in particular,a method of applying the coating solution to a base material and thendrying the base material for formation of a porous layer. Thisarrangement makes it possible to produce a porous layer having a smallerporosity variation and fewer wrinkles.

The above drying can be carried out with the use of a normal dryingdevice.

The porous layer normally has, on one surface of the porous basematerial, an applied amount (weight per unit area) within a range ofpreferably 0.5 g/m² to 20 g/m², more preferably 0.5 g/m² to 10 g/m²,preferably 0.5 g/m² to 1.5 g/m², in terms of the solid content in viewof adhesiveness to an electrode and ion permeability. This means thatthe amount of the coating solution to be applied to the porous basematerial is preferably adjusted so that the porous layer in a laminatedbody or nonaqueous electrolyte secondary battery separator to beproduced has an applied amount (weight per unit area) within the aboverange.

In a case where an additional layer such as a heat-resistant layer is tobe disposed on the laminated body, such a heat-resistant layer can bedisposed by a method similar to the above method except that the resinfor the porous layer is replaced with a resin for the heat-resistantlayer.

The present embodiment is arranged such that in any of the processes (1)to (3), changing the amount of resin for a porous layer which resin isto be dissolved or dispersed in a solution can adjust the volume ofresin that is contained per square meter of a porous layer havingundergone immersion in an electrolyte solution and that has absorbed theelectrolyte solution.

Further, changing the amount of solvent in which the resin for theporous layer is to be dissolved or dispersed can adjust the porosity andaverage pore diameter of a porous layer having undergone immersion in anelectrolyte solution.

<Method for Controlling Crystal Forms of PVDF-Based Resin>

A laminated body in accordance with the present embodiment is producedwhile adjustment is made of the drying conditions (for example, thedrying temperature, and the air velocity and direction during drying)and/or the deposition temperature (that is, the temperature at which aporous layer containing a PVDF-based resin is deposited with use of adeposition solvent or a low-boiling-point organic acid) for theabove-described method to control the crystal forms of the PVDF-basedresin to be contained in a porous layer to be formed. Specifically, alaminated body of the present embodiment can be produced while thedrying conditions and the deposition temperature are adjusted so thatthe PVDF-based resin contains crystal form α in an amount of not lessthan 36 mol % (preferably not less than 63 mol %, more preferably notless than 70 mol %, further preferably not less than 80 mol %;preferably not more than 95 mol %) with respect to 100 mol % of thetotal amount of the crystal form α and crystal form β contained.

The drying conditions and the deposition temperature, which are adjustedso that the PVDF-based resin contains crystal form α in an amount of notless than 36 mol % with respect to 100 mol % of the total amount of thecrystal form α and crystal form β contained, may be changed asappropriate in correspondence with, for example, the method forproducing a porous layer, the kind of solvent (dispersion medium) to beused, the kind of deposition solvent to be used, and/or the kind oflow-boiling-point organic acid to be used.

In a case where a deposition solvent is not used and the coatingsolution is simply dried as in the process (1), the drying conditionsmay be changed as appropriate in correspondence with, for example, theamount of the solvent in the coating solution, the concentration of thePVDF-based resin in the coating solution, the amount of the filler (ifcontained), and/or the amount of the coating solution to be applied. Ina case where a porous layer is to be formed through the process (1)described above, it is preferable that the drying temperature be 30° C.to 100° C., that the direction of hot air for drying be perpendicular toa porous base material or electrode sheet to which the coating solutionhas been applied, and that the velocity of the hot air be 0.1 m/s to 40m/s. Specifically, in a case where a coating solution to be appliedcontains N-methyl-2-pyrrolidone as the solvent for dissolving aPVDF-based resin, 1.0% by mass of a PVDF-based resin, and 9.0% by massof alumina as an inorganic filler, the drying conditions are preferablyadjusted so that the drying temperature is 40° C. to 100° C., that thedirection of hot air for drying is perpendicular to a porous basematerial or electrode sheet to which the coating solution has beenapplied, and that the velocity of the hot air is 0.4 m/s to 40 m/s.

In a case where a porous layer is to be formed through the process (2)described above, it is preferable that the deposition temperature be−25° C. to 60° C. and that the drying temperature be 20° C. to 100° C.Specifically, in a case where a porous layer is to be formed through theabove-described process (2) with use of N-methylpyrrolidone as thesolvent for dissolving a PVDF-based resin and isopropyl alcohol as thedeposition solvent, it is preferable that the deposition temperature be−10° C. to 40° C. and that the drying temperature be 30° C. to 80° C.

<Nonaqueous Electrolyte Secondary Battery Member and NonaqueousElectrolyte Secondary Battery>

A nonaqueous electrolyte secondary battery in accordance with thepresent embodiment includes the above laminated body as a separator.More specifically, a nonaqueous electrolyte secondary battery inaccordance with the present embodiment includes a member for anonaqueous electrolyte secondary battery (hereinafter referred to as a“nonaqueous electrolyte secondary battery member”) which member includesa cathode, the above laminated body, and an anode that are arranged inthis order. The nonaqueous electrolyte secondary battery member is alsoencompassed in the scope of the present invention. The description belowdeals with a lithium-ion secondary battery as an example of thenonaqueous electrolyte secondary battery. The components of thenonaqueous electrolyte secondary battery other than a separator are notlimited to those described below.

A nonaqueous electrolyte secondary battery in accordance with thepresent embodiment can include a nonaqueous electrolyte containing, forexample, an organic solvent and a lithium salt dissolved therein.Examples of the lithium salt include LiClO₄, LiPF₆, LiAsF₆, LiSbF₆,LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, lower aliphaticcarboxylic acid lithium salt, and LiAlCl₄. The present embodiment mayuse only one kind of the above lithium salts or two or more kinds of theabove lithium salts in combination.

It is preferable to use, among the above lithium salts, at least onefluorine-containing lithium salt selected from the group consisting ofLiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, and LiC(CF₃SO₂)₃.

Specific examples of the organic solvent in the nonaqueous electrolyteinclude carbonates such as ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate,4-trifluoromethyl-1,3-dioxolane-2-one, and 1,2-di(methoxycarbonyloxy)ethane; ethers such as 1,2-dimethoxyethane,1,3-dimethoxypropane, pentafluoropropyl methylether,2,2,3,3-tetrafluoropropyl difluoro methylether, tetrahydrofuran, and2-methyl tetrahydrofuran; esters such as methyl formate, methyl acetate,and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile;amides such as N,N-dimethylformamide and N,N-dimethylacetamide;carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compoundssuch as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone; andfluorine-containing organic solvents prepared by introducing a fluorinegroup into the organic solvents described above. The present embodimentmay use only one kind of the above organic solvents or two or more kindsof the above organic solvents in combination.

Among the above organic solvents, carbonates are preferable. A mixedsolvent of a cyclic carbonate and an acyclic carbonate or a mixedsolvent of a cyclic carbonate and one of ethers is further preferable.

The mixed solvent of a cyclic carbonate and an acyclic carbonate isfurther preferably a mixed solvent of ethylene carbonate, dimethylcarbonate, and ethyl methyl carbonate because such a mixed solventallows a wider operating temperature range, and is not easily decomposedeven in a case where the present embodiment uses, as an anode activematerial, a graphite material such as natural graphite or artificialgraphite.

The cathode is normally a sheet-shaped cathode including (i) a cathodemix containing a cathode active material, an electrically conductivematerial, and a binder and (ii) a cathode current collector supportingthe cathode mix thereon.

The cathode active material is, for example, a material capable of beingdoped and dedoped with lithium ions. Specific examples of such amaterial include a lithium complex oxide containing at least onetransition metal such as V, Mn, Fe, Co, or Ni.

Among such lithium complex oxides, (i) a lithium complex oxide having anα-NaFeO₂ structure such as lithium nickelate and lithium cobaltate and(ii) a lithium complex oxide having a spinel structure such as lithiummanganese spinel are preferable because such lithium complex oxides havea high average discharge potential. The lithium complex oxide mayfurther contain any of various metallic elements, and is furtherpreferably complex lithium nickelate.

Further, the complex lithium nickelate particularly preferably containsat least one metallic element selected from the group consisting of Ti,Zr, Ce, Y, V, Cr, Mn, Fe, Co, Cu, Ag, Mg, Al, Ga, In, and Sn at aproportion of 0.1 mol % to 20 mol % with respect to the sum of thenumber of moles of the at least one metallic element and the number ofmoles of Ni in the lithium nickelate. This is because such a complexlithium nickelate allows an excellent cycle characteristic for use in ahigh-capacity battery. Among others, an active material that contains Alor Mn and that contains Ni at a proportion of not less than 85 mol %,further preferably not less than 90 mol %, is particularly preferablebecause a nonaqueous electrolyte secondary battery including a cathodecontaining the above active material has an excellent cyclecharacteristic for use as a high-capacity battery. Al or Mn is containedat a proportion of 0.1 mol % to 20 mol %, and Ni is contained at aproportion of not less than 85 mol %, further preferably not less than90 mol %, with respect to the sum of the number of moles of Al or Mn andthe number of moles of Ni in the lithium nickelate. Further, the totalof mol % of Al or Mn and mol % of Ni is 100 mol %.

Examples of the electrically conductive material include carbonaceousmaterials such as natural graphite, artificial graphite, cokes, carbonblack, pyrolytic carbons, carbon fiber, and a fired product of anorganic polymer compound. The present embodiment may use (i) only onekind of the above electrically conductive materials or (ii) two or morekinds of the above electrically conductive materials in combination, forexample a mixture of artificial graphite and carbon black.

Examples of the binder include thermoplastic resins such aspolyvinylidene fluoride, a copolymer of vinylidene fluoride,polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylenecopolymer, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,an ethylene-tetrafluoroethylene copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, athermoplastic polyimide, polyethylene, and polypropylene; an acrylicresin; and styrene-butadiene rubber. The binder functions also as athickening agent.

The cathode mix may be prepared by, for example, a method of applyingpressure to the cathode active material, the electrically conductivematerial, and the binder on the cathode current collector or a method ofusing an appropriate organic solvent so that the cathode activematerial, the electrically conductive material, and the binder are in apaste form.

Examples of the cathode current collector include electric conductorssuch as Al, Ni, and stainless steel. Among these, Al is preferable as itis easy to process into a thin film and less expensive.

The sheet-shaped cathode may be produced, that is, the cathode mix maybe supported by the cathode current collector, through, for example, amethod of applying pressure to the cathode active material, theelectrically conductive material, and the binder on the cathode currentcollector to form a cathode mix thereon or a method of (i) using anappropriate organic solvent so that the cathode active material, theelectrically conductive material, and the binder are in a paste form toprovide a cathode mix, (ii) applying the cathode mix to the cathodecurrent collector, (iii) drying the applied cathode mix to prepare asheet-shaped cathode mix, and (iv) applying pressure to the sheet-shapedcathode mix so that the sheet-shaped cathode mix is firmly fixed to thecathode current collector.

The anode is normally a sheet-shaped anode including (i) an anode mixcontaining an anode active material and (ii) an anode current collectorsupporting the anode mix thereon. The sheet-shaped anode preferablycontains the above electrically conductive material and binder.

The anode active material is, for example, (i) a material capable ofbeing doped and dedoped with lithium ions, (ii) a lithium metal, or(iii) a lithium alloy. Specific examples of the material includecarbonaceous materials such as natural graphite, artificial graphite,cokes, carbon black, pyrolytic carbons, carbon fiber, and a firedproduct of an organic polymer compound; chalcogen compounds such as anoxide and a sulfide that are doped and dedoped with lithium ions at anelectric potential lower than that for the cathode; metals such asaluminum (Al), lead (Pb), tin (Sn), bismuth (Bi), or silicon (Si), eachof which is alloyed with alkali metal; an intermetallic compound (AlSb,Mg₂Si, NiSi₂) of a cubic system in which intermetallic compound alkalimetal can be inserted in voids in a lattice; and a lithium nitrogencompound (Li₃-xM_(x)N (where M represents a transition metal)).

Among the above anode active materials, a carbonaceous materialcontaining a graphite material such as natural graphite or artificialgraphite as a main component is preferable, an anode active materialwhich is a mixture of a graphite material and silicon and in which theratio of Si to C is not less than 5% is more preferable, and an anodeactive material in which the ratio of Si to C is not less than 10% isfurther preferable. This is because such a carbonaceous material hashigh electric potential flatness and low average discharge potential andcan thus be combined with a cathode to achieve high energy density. Theanode active material, in other words, contains Si at a proportion ofmore preferably not less than 5 mol %, further preferably not less than10 mol %, with respect to the sum (100 mol %) of the number of moles ofC in the graphite material and the number of moles of Si.

The anode mix may be prepared by, for example, a method of applyingpressure to the anode active material on the anode current collector ora method of using an appropriate organic solvent so that the anodeactive material is in a paste form.

The anode current collector is, for example, Cu, Ni, or stainless steel.Among these, Cu is preferable as it is not easily alloyed with lithiumin the case of a lithium-ion secondary battery in particular and iseasily processed into a thin film.

The sheet-shaped anode may be produced, that is, the anode mix may besupported by the anode current collector, through, for example, a methodof applying pressure to the anode active material on the anode currentcollector to form an anode mix thereon or a method of (i) using anappropriate organic solvent so that the anode active material is in apaste form to provide an anode mix, (ii) applying the anode mix to theanode current collector, (iii) drying the applied anode mix to prepare asheet-shaped anode mix, and (iv) applying pressure to the sheet-shapedanode mix so that the sheet-shaped anode mix is firmly fixed to theanode current collector. The above paste preferably includes aconductive aid and binder.

A nonaqueous electrolyte secondary battery in accordance with thepresent embodiment may be produced by (i) arranging the cathode, thelaminated body, and the anode in this order to form a nonaqueouselectrolyte secondary battery member, (ii) inserting the nonaqueouselectrolyte secondary battery member into a container that is for use asa housing of the nonaqueous electrolyte secondary battery, (iii) fillingthe container with a nonaqueous electrolyte, and (iv) hermeticallysealing the container under reduced pressure. The nonaqueous electrolytesecondary battery may have any shape such as the shape of a thin plate(sheet), a disk, a cylinder, or a prism such as a cuboid. The nonaqueouselectrolyte secondary battery may be produced by any method, and may beproduced by a conventionally publicly known method.

A nonaqueous electrolyte secondary battery in accordance with thepresent embodiment includes, as a separator, a laminated body including(i) a porous base material having a ΔWI value of not more than 2.5 asdescribed above and (ii) the above-described porous layer. The laminatedbody contains only a small amount of oxide of resin, which oxide cancause side reactions during charging and discharging. The nonaqueouselectrolyte secondary battery can thus exhibit an excellent cyclecharacteristic. In Production Examples described later, for example, anonaqueous electrolyte secondary battery including the porous basematerial showed a high discharge capacity maintaining ratio of more than70.0%. A nonaqueous electrolyte secondary battery including thelaminated body should also show a similar discharge capacity maintainingratio.

The discharge capacity maintaining ratio is the ratio of (i) thedischarge capacity that a nonaqueous electrolyte secondary battery hasafter the nonaqueous electrolyte secondary battery, which had not beensubjected to a charge-and-discharge cycle, was subjected topredetermined charge-and-discharge cycles to (ii) the initial dischargecapacity. A higher discharge capacity maintaining ratio means a moreexcellent cycle characteristic, that is, a longer life for the battery.Note that a method for calculating the discharge capacity maintainingratio will be described later in the Examples section.

In a case where a discharge capacity maintaining ratio calculated inaccordance with Formula (3) described later in <Method for measuringvarious physical properties, etc. of porous base material> is not lessthan 70.0%, the battery has a long life and thus has a sufficient cyclecharacteristic. A nonaqueous electrolyte secondary battery member of thepresent embodiment and a nonaqueous electrolyte secondary battery of thepresent embodiment each include the above-described porous layer, whichcontains a polyvinylidene fluoride-based resin (PVDF-based resin), thePVDF-based resin containing crystal form α in an amount of not less than36 mol % with respect to 100 mol % of the total amount of the crystalform α and crystal form β contained. The nonaqueous electrolytesecondary battery member in accordance with the present embodiment andthe nonaqueous electrolyte secondary battery in accordance with thepresent invention are not easily curled as a result.

The present invention is not limited to the embodiments, but can bealtered by a skilled person in the art within the scope of the claims.The present invention also encompasses, in its technical scope, anyembodiment derived by combining technical means disclosed in differingembodiments. Further, it is possible to form a new technical feature bycombining the technical means disclosed in the respective embodiments.

EXAMPLES

The following description will discuss an embodiment of the presentinvention in greater detail with reference to Examples and ComparativeExample. Note, however, that the present invention is not limited to theExamples and Comparative Example below.

<Method for Measuring Various Physical Properties, etc. of Porous BaseMaterial>

The physical properties and the like of separators and porous layers inthe Examples and Comparative Examples were measured by methods describedbelow.

(1) Film Thickness (Unit: μm)

The film thickness was measured with use of a high-accuracy digitallength measuring machine available from Mitsutoyo Corporation.

(2) Porosity (Unit: %)

An 8 cm square was cut from a film, and the weight W (g) and thickness D(cm) of the square were measured. The respective weights of materials inthe sample were calculated. The weight Wi (g) of each material wasdivided by the absolute specific gravity for calculation of the volumeof the material. The porosity (volume %) of the sample was determined inaccordance with the following formula:

Porosity(volume %)=100−[{(W1/absolute specific gravity 1)+(W2/absolutespecific gravity 2)+ . . . +(Wn/absolute specific gravityn)}/(8×8×D)]×100

(3) Weather Resistance Test

A test piece of each of the porous base materials (separators) producedin the Production Examples and Comparative Production Examples wasirradiated with ultraviolet light with use of Sunshine Weather Meter S80(available from Suga Tester) in conformity with JIS B 7753.Specifically, the test piece was irradiated with ultraviolet light withan intensity of 255 W/m² for 75 hours at a relative humidity of 50% withuse of (i) a sunshine carbon arc light source (with four pairs of ultralong-life carbon rods) set to have a discharge voltage of 50 V and adischarge current of 60 A and (ii) a black panel having a temperature of60° C.

(4) Measurement of White Index (WI)

The WI value of each separator was measured by Specular ComponentIncluded (SCI) method (including specular reflection) with use of aspectrocolorimeter (CM-2002, available from MINOLTA). During themeasurement of the WI value, the separator was placed on black paper(available from Hokuetsu Kishu Paper Co., Ltd., colored high-qualitypaper, black, thickest type, shirokuhan (788 mm×1091 mm with the longside extending in a machine direction)). WI measurements were made (WI₀and WI₁, respectively) before and after the separator was subjected tothe above weather resistance test, and ΔWI was then determined inaccordance with Formula (1) above.

(5) Measurement of Discharge Capacity Maintaining Ratio

Nonaqueous electrolyte secondary batteries each of which had not beensubjected to a charge-and-discharge cycle were each subjected to threecycles of initial charge and discharge. Each of the three cycles of theinitial charge and discharge was carried out at 25° C., at a voltageranging from 4.1 V to 2.7 V, and at an electric current value of 0.2 C.Note that the value of an electric current at which a battery ratedcapacity defined as a one-hour rate discharge capacity is discharged inone hour is assumed to be 1 C. This applies also to the followingdescriptions. Subsequently, the nonaqueous electrolyte secondarybatteries were each subjected to three cycles of charge and discharge ateach of the electric current values of 1 C, 5 C, 10 C, and 20 C.

Finally, the nonaqueous electrolyte secondary batteries were eachsubjected to three cycles of charge and discharge at an electric currentvalue of 0.2 C. The discharge capacity maintaining ratio after 18 cycleswas calculated in accordance with the following Formula (2):

Discharge capacity maintaining ratio(%) after 18 cycles=(Dischargecapacity at 0.2 C in 18th cycle/discharge capacity at 0.2 C in firstcycle)×100   (2)

Further, the discharge capacity maintaining ratio after 180 cycles wascalculated in accordance with the following Formula (3):

Discharge capacity maintaining ratio(%) after 180 cycles=(Dischargecapacity at 0.2 C in 18th cycle/discharge capacity at 0.2 C in firstcycle)¹⁰×100   (3)

PRODUCTION EXAMPLES

<Production of Separator>

Production Example 1

Polyethylene powder (ultra-high molecular weight polyethylene GUR2024[available from Ticona Corporation]; weight-average molecular weight:4,970,000) and low molecular weight polyethylene powder (polyethylenewax FNP-0115 [available from Nippon Seiro Co., Ltd.]) having aweight-average molecular weight of 1000 were mixed for preparation of aresin mixture containing the polyethylene powder at a proportion of 68%by weight and the low molecular weight polyethylene powder at aproportion of 32% by weight. Then, 100 parts by weight of the resinmixture, calcium carbonate (available from Maruo Calcium Co., Ltd.;average particle diameter: 0.10 μm) at a proportion of 160 parts byweight with respect to 100 parts by weight of the resin mixture, and anantioxidant at a proportion of 3 parts by weight (IRG1010 [availablefrom Ciba Specialty Chemicals]/Irf168 [available from Ciba SpecialtyChemicals]=2 parts by weight/1 part by weight) were mixed. The resultingmixture was melted and kneaded at 200° C. with use of a twin screwkneading extruder. This produced a resin composition.

The resin composition was extruded from a T-die having a temperature of240° C. into a sheet shape, and the resulting product was rolled withuse of a pair of rolls each having a surface temperature of 150° C. Thisprepared a sheet. The resin composition was exposed to air for 3.6seconds (after the resin composition was discharged from the T-die untilit was cooled with the rolls [that is, until it came into contact withthe rolls]). In all of the Production Examples and ComparativeProduction Examples, the same extruder was used, and the distancebetween the T-die and the rolls (corresponding to the distance 4 shownin FIG. 1) was 15 cm.

This sheet was immersed in an aqueous hydrochloric acid solution(containing 4 mol/L of hydrochloric acid and 1% by weight of nonionicsurfactant [SANMORIN 11, available from Sanyo Chemical Industries,Ltd.]) having a temperature of 40° C. for removal of the calciumcarbonate. Subsequently, the sheet was stretched 6-fold at 100° C. withuse of a tenter uniaxial stretching machine available from Ichikin Co.,Ltd. This produced a separator 1 (polyolefin porous base material).

Production Example 2

Polyethylene powder (ultra-high molecular weight polyethylene GUR4032[available from Ticona Corporation]; weight-average molecular weight:4,970,000) and low molecular weight polyethylene powder (polyethylenewax FNP-0115 [available from Nippon Seiro Co., Ltd.]) were mixed forpreparation of a resin mixture containing the polyethylene powder at aproportion of 70% by weight and the low molecular weight polyethylenepowder at a proportion of 30% by weight. Then, 100 parts by weight ofthe resin mixture, calcium carbonate (available from Maruo Calcium Co.,Ltd.; average particle diameter: 0.10 μm) at a proportion of 160 partsby weight with respect to 100 parts by weight of the resin mixture, andan antioxidant at a proportion of 3 parts by weight (IRG1010/Irf168=2parts by weight/1 part by weight) were mixed. The resulting mixture wasmelted and kneaded at 200° C. with use of a twin screw kneadingextruder. This produced a resin composition.

The resin composition was extruded from a T-die having a temperature of247° C. into a sheet shape, and the resulting product was rolled withuse of a pair of rolls each having a surface temperature of 150° C. Thisprepared a sheet. The resin composition was exposed to air for 3.0seconds (after the resin composition was discharged from the T-die untilit was cooled with the rolls).

This sheet was immersed in an aqueous hydrochloric acid solution(containing 4 mol/L of hydrochloric acid and 6% by weight of nonionicsurfactant) having a temperature of 40° C. for removal of the calciumcarbonate. Subsequently, the sheet was stretched 6-fold at 105° C. withuse of a tenter uniaxial stretching machine available from Ichikin Co.,Ltd. This produced a separator 2 (polyolefin porous base material).

Comparative Production Example 1

Polyethylene powder (ultra-high molecular weight polyethylene GUR4032[available from Ticona Corporation]; weight-average molecular weight:4,970,000) and low molecular weight polyethylene powder (polyethylenewax FNP-0115 [available from Nippon Seiro Co., Ltd.]) were mixed forpreparation of a resin mixture containing the polyethylene powder at aproportion of 71% by weight and the low molecular weight polyethylenepowder at a proportion of 29% by weight. Then, 100 parts by weight ofthe resin mixture, calcium carbonate (available from Maruo Calcium Co.,Ltd.; average particle diameter: 0.10 μm) at a proportion of 160 partsby weight with respect to 100 parts by weight of the resin mixture, andan antioxidant at a proportion of 3 parts by weight (IRG1010/Irf168=2parts by weight/1 part by weight) were mixed. The resulting mixture wasmelted and kneaded at 200° C. with use of a twin screw kneadingextruder. This produced a resin composition.

The resin composition was extruded from a T-die having a temperature of253° C. into a sheet shape, and the resulting product was rolled withuse of a pair of rolls each having a surface temperature of 150° C. Thisprepared a sheet. The resin composition was exposed to air for 2.3seconds (after the resin composition was discharged from the T-die untilit was cooled with the rolls).

This sheet was immersed in an aqueous hydrochloric acid solution(containing 4 mol/L of hydrochloric acid and 1% by weight of nonionicsurfactant) having a temperature of 40° C. for removal of the calciumcarbonate. Subsequently, the sheet was stretched 7-fold at 100° C. withuse of a tenter uniaxial stretching machine available from Ichikin Co.,Ltd. This produced a comparative separator 1 (polyolefin porous basematerial).

Comparative Production Example 2

Polyethylene powder (ultra-high molecular weight polyethylene GUR4032[available from Ticona Corporation]; weight-average molecular weight:4,970,000) and low molecular weight polyethylene powder (polyethylenewax FNP-0115 [available from Nippon Seiro Co., Ltd.]) were mixed forpreparation of a resin mixture containing the polyethylene powder at aproportion of 70% by weight and the low molecular weight polyethylenepowder at a proportion of 30% by weight. Then, 100 parts by weight ofthe resin mixture, calcium carbonate (available from Maruo Calcium Co.,Ltd.; average particle diameter: 0.10 μm) at a proportion of 160 partsby weight with respect to 100 parts by weight of the resin mixture, andan antioxidant at a proportion of 3 parts by weight (IRG1010/Irf168=2parts by weight/1 part by weight) were mixed. The resulting mixture wasmelted and kneaded at 200° C. with use of a twin screw kneadingextruder. This produced a resin composition.

The resin composition was extruded from a T-die having a temperature of252° C. into a sheet shape, and the resulting product was rolled withuse of a pair of rolls each having a surface temperature of 150° C. Thisprepared a sheet. The resin composition was exposed to air for 3.6seconds (after the resin composition was discharged from the T-die untilit was cooled with the rolls).

This sheet was immersed in an aqueous hydrochloric acid solution(containing 4 mol/L of hydrochloric acid and 6% by weight of nonionicsurfactant) having a temperature of 40° C. for removal of the calciumcarbonate. Subsequently, the sheet was stretched 6-fold at 105° C. withuse of a tenter uniaxial stretching machine available from Ichikin Co.,Ltd. This produced a comparative separator 2 (polyolefin porous basematerial).

<Preparation of Nonaqueous Electrolyte Secondary Battery>

Next, nonaqueous electrolyte secondary batteries were produced by thefollowing method with use of the separators 1 and 2 and comparativeseparators 1 and 2, which were prepared in the Production Examples andComparative Production Examples.

(Cathode)

A commercially available cathode produced by applying, to an aluminumfoil, a mixture of 92 parts by weight ofLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂(cathode active material), 5 parts by weightof an electrically conductive material, and 3 parts by weight ofpolyvinylidene fluoride (PVDF) was used to prepare a nonaqueouselectrolyte secondary battery. The aluminum foil was partially cut offso that a cathode active material layer was present in an area of 40mm×35 mm and that that area was surrounded by an area with a width of 13mm in which area no cathode active material layer was present. Thecutoff was used as a cathode. The cathode active material layer had athickness of 58 μm and a density of 2.50 g/cm³.

(Anode)

A commercially available anode was used that was produced by applying,to a copper foil, a mixture of 98 parts by weight of graphite as ananode active material, 1 part by weight of a styrene-1,3-butadienecopolymer, and 1 part by weigh of sodium carboxymethylcellulose. Thecopper foil was partially cut off so that an anode active material layerwas present in an area of 50 mm×40 mm and that that area was surroundedby an area with a width of 13 mm in which area no anode active materiallayer was present. The cutoff was used as an anode. The anode activematerial layer had a thickness of 49 μm and a density of 1.40 g/cm³.

(Assembly)

The cathode, the separator (separator 1 or 2, or comparative separator 1or 2), and the anode were laminated (disposed) in this order in alaminate pouch, so that a nonaqueous electrolyte secondary batterymember was obtained. During this operation, the cathode and the anodewere arranged so that the cathode active material layer of the cathodehad a main surface that was entirely covered by the main surface of theanode active material layer of the anode.

Subsequently, the nonaqueous electrolyte secondary battery member wasput into a bag made of a laminate of an aluminum layer and a heat seallayer. Further, 0.25 mL of nonaqueous electrolyte was put into the bag.The nonaqueous electrolyte was an electrolyte at 25° C. prepared bydissolving LiPF₆ with a concentration of 1.0 mole per liter in a mixedsolvent of ethyl methyl carbonate, diethyl carbonate, and ethylenecarbonate in a volume ratio of 50:20:30. Then, the bag was heat-sealedwhile the pressure inside the bag was reduced, so that nonaqueouselectrolyte secondary batteries 1 and 2 (which included the separators 1and 2, respectively) and comparative nonaqueous electrolyte secondarybatteries 1 and 2 (which included the comparative separators 1 and 2,respectively) were prepared.

Production Examples 1 and 2 and Comparative Examples 1 and 2

In Production Examples 1 and 2, the discharge capacity maintaining ratioafter 180 cycles was determined of each of the nonaqueous electrolytesecondary batteries 1 and 2 (which included the separators 1 and 2,respectively). In Comparative Examples 1 and 2, the discharge capacitymaintaining ratio after 180 cycles was determined of each of thecomparative nonaqueous electrolyte secondary batteries 1 and 2 (whichincluded the comparative separators 1 and 2, respectively). Table 1shows the results.

In Table 1, the items “Extrusion temperature”, “Air exposure timeperiod”, and “Surfactant concentration” indicate conditions under whicha separator was produced in each of the Production Examples andComparative Production Examples, the conditions being described aboveunder [Production Examples]. The item “Surfactant concentration” refersto the concentration of a nonionic surfactant in an aqueous hydrochloricacid solution.

In Table 1, the items “Film thickness”, “Porosity” and “ΔWI” indicatethe thickness, porosity, and ΔWI that the separator of each of theProduction Examples and Comparative Examples was designed to have.

TABLE 1 Extrusion Air exposure Surfactant Discharge capacity temperaturetime period concentration Film thickness Porosity maintaining ratio (%)(° C.) (seconds) (wt %) (μm) (%) ΔW1 after 180 cycles Production 240 3.61 11 37 2.19 70.7 Example 1 Production 247 3.0 6 16 52 2.16 70.9 Example2 Comparative 253 2.3 1 12 50 3.26 69.4 Example 1 Comparative 252 3.6 616 65 2.79 67.8 Example 2

Table 1 shows that the nonaqueous electrolyte secondary batteries 1 and2 (which included the separators 1 and 2, respectively, each having aΔWI value of not more than 2.5) each had a discharge capacitymaintaining ratio after 180 cycles of more than 70.0%, indicating a goodcycle characteristic. In contrast, the comparative nonaqueouselectrolyte secondary batteries 1 and 2 (which included the comparativeseparators 1 and 2, respectively, each having a ΔWI value of more than2.5) each had a discharge capacity maintaining ratio after 180 cycles ofless than 70.0%. Such a discharge capacity maintaining ratio of lessthan 70.0% indicates an insufficient cycle characteristic for thebattery to have a long life.

As shown in Table 1, the temperature at which the resin was extrudedfrom the T-die was 240° C. and 247° C. in Production Examples 1 and 2,respectively, and 253° C. and 252° C. in Comparative Production Examples1 and 2, respectively.

Production Examples 1 and 2 each used a separator containing resinhaving been exposed to air at a temperature lower than the temperatureat which resin was exposed to air in Comparative Examples 1 and 2 asdescribed above. Such separators 1 and 2 each had a ΔWI value of notmore than 2.5, which presumably allowed the nonaqueous electrolytesecondary batteries 1 and 2 to each exhibit an excellent cyclecharacteristic.

Comparative Production Examples 1 and 2, on the other hand, each used anextrusion temperature of higher than 250° C. This should have (i)increased the amount of oxide in resin after the resin was extrudeduntil it came into contact with rolls and (ii) lead the producedseparators to each have a ΔWI value of more than 2.5, with the resultthat the nonaqueous electrolyte secondary batteries 1 and 2 (whichincluded one of the separators) each had an insufficient dischargecapacity maintaining ratio of less than 70.0%.

While Comparative Production Example 1 had an air exposure time periodof 2.3 seconds, which was the shortest among all the Production Examplesand Comparative Production Examples, Comparative Production Example 1used an extrusion temperature of 253° C. A high extrusion temperature ofhigher than 250° C. presumably more than offset the advantage of theshort air exposure time period.

[Various Methods for Measuring Physical Properties of Laminated Body]

In Examples 1 to 7 and Comparative Examples 3 and 4 below, propertiessuch as the α rate and curl property were measured by the followingmethods:

(6) Method for Calculating α Rate

An α rate (%) was measured by the method below, the a rate (%) being amolar ratio (%) of crystal form α in the PVDF-based resin contained inthe porous layer in the laminated body produced in each of Examples 1 to7 and Comparative Examples 3 and 4 below with respect to the totalamount of the crystal form α and crystal form β contained in thePVDF-based resin.

An 80 mm×80 mm square was cut out from the laminated body. The infraredabsorption spectrum of the cutout at wave numbers from 4000 cm⁻¹ to 400cm⁻¹ (measurement range) was measured at room temperature (approximately25° C.) with use of an FT-IR spectrometer (available from Bruker OpticsK.K.; model: ALPHA Platinum-ATR) with a resolution of 4 cm⁻¹ and 512times of scanning. Then, from the spectrum measured, the absorptionintensity at 765 cm⁻¹ (characteristic absorption of crystal form α) andthe absorption intensity at 840 cm⁻¹ (characteristic absorption ofcrystal form β) were determined. The starting point and end point of awaveform formed with the wave number set as a peak were connected with astraight line, where the length between the straight line and the peakwave number (peak top) denoted an absorption intensity. For crystal formα, a maximum possible absorption intensity within the wave number rangeof 775 cm⁻¹ to 745 cm⁻¹ was assumed to be the absorption intensity at765 cm. For crystal form β, a maximum possible absorption intensitywithin the wave number range of 850 cm⁻¹ to 815 cm⁻¹ was assumed to bethe absorption intensity at 840 cm⁻¹.

The α rate was calculated as described above in accordance with theFormula (4a) below on the basis of a value obtained by (i) determiningthe absorption intensity at 765 cm⁻¹ corresponding to crystal form α andthe absorption intensity at 840 cm⁻¹ corresponding to crystal form β and(ii) multiplying the absorption intensity of crystal form α by 1.681(correction factor) with reference to Japanese Patent ApplicationPublication, Tokukai, No. 2005-200623.

α rate(%)=[1−{absorption intensity at 840 cm⁻¹/(absorption intensity at765 cm⁻¹×correction factor (1.68 1)+absorption intensity at 840cm⁻¹)}]×100   (4a)

(7) Curl Measurement

An 8 cm×8 cm square was cut out from the laminated body. The cutout waskept at room temperature (approximately 25° C.) and at a dew point of−30° C. for one (1) day. The appearance of the cutout was then evaluatedon the basis of the following criterion: The rate “C” represents a stateof a complete curl, the rates “A” and “B” each represent a better state,and the rate “A” represents the most preferable state.

-   -   A: No curved ends    -   B: Although an end(s) is curved, the remaining portion is mostly        not curved and is flat.    -   C: Opposite ends curved into a tube shape

Example 1

An N-methyl-2-pyrrolidone (hereinafter referred to also as “NMP”)solution (available from Kureha Corporation; product name: L#9305,weight-average molecular weight: 1,000,000) containing a PVDF-basedresin (polyvinylidene fluoride-hexafluoropropylene copolymer) wasprepared as a coating solution. The coating solution was applied by adoctor blade method to the porous base material produced in ProductionExample 1 so that the applied coating solution weighed 6.0 g per squaremeter of the PVDF-based resin in the coating solution. The porous film,to which the coating solution had been applied, was immersed into2-propanol while the coating film was wet with the solvent, and was thenleft to stand still at 25° C. for 5 minutes. This produced a laminatedporous base material (1-i). The laminated porous base material (1-i)produced was further immersed into other 2-propanol while the laminatedporous base material (1-i) was wet with the above immersion solvent, andwas then left to stand still at 25° C. for 5 minutes. This produced alaminated porous base material (1-ii). The laminated porous basematerial (1-ii) produced was dried at 65° C. for 5 minutes. Thisproduced a laminated body (1). Table 2 shows the results of evaluationof the laminated body (1) produced.

Example 2

A laminated body (2) was prepared by a method similar to the method usedin Example 1 except that the porous base material prepared in ProductionExample 2 was used. Table 2 shows the results of evaluation of thelaminated body (2) produced.

Example 3

A porous film to which a coating solution had been applied as in Example1 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at 0° C. for 5 minutes. Thisproduced a laminated porous base material (3-i). The laminated porousbase material (3-i) produced was further immersed into other 2-propanolwhile the laminated porous base material (3-i) was wet with the aboveimmersion solvent, and was then left to stand still at 25° C. for 5minutes. This produced a laminated porous base material (3-ii). Thelaminated porous base material (3-ii) produced was dried at 30° C. for 5minutes. This produced a laminated body (3). Table 2 shows the resultsof evaluation of the laminated body (3) produced.

Example 4

A porous film to which a coating solution had been applied as in Example2 was treated by a method similar to the method used in Example 3. Thisproduced a laminated body (4). Table 2 shows the results of evaluationof the laminated body (4) produced.

Example 5

A porous film to which a coating solution had been applied as in Example1 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at −5° C. for 5 minutes. Thisproduced a laminated porous base material (5-i). The laminated porousbase material (5-i) produced was further immersed into other 2-propanolwhile the laminated porous base material (5-i) was wet with the aboveimmersion solvent, and was then left to stand still at 25° C. for 5minutes. This produced a laminated porous base material (5-ii). Thelaminated porous base material (5-ii) produced was dried at 30° C. for 5minutes. This produced a laminated body (5). Table 2 shows the resultsof evaluation of the laminated body (5) produced.

Example 6

A porous film to which a coating solution had been applied as in Example2 was treated by a method similar to the method used in Example 5. Thisproduced a laminated body (6). Table 2 shows the results of evaluationof the laminated body (6) produced.

Example 7

A porous film to which a coating solution had been applied as in Example2 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at −10° C. for 5 minutes. Thisproduced a laminated porous base material (7-i). The laminated porousbase material (7-i) produced was further immersed into other 2-propanolwhile the laminated porous base material (7-i) was wet with the aboveimmersion solvent, and was then left to stand still at 25° C. for 5minutes. This produced a laminated porous base material (7-ii). Thelaminated porous base material (7-ii) produced was dried at 30° C. for 5minutes. This produced a laminated body (7). Table 2 shows the resultsof evaluation of the laminated body (7) produced.

Comparative Example 3

A porous film to which a coating solution had been applied as in Example1 was immersed into 2-propanol while the coating film was wet with thesolvent, and was then left to stand still at −78° C. for 5 minutes. Thisproduced a laminated porous base material (8-i). The laminated porousbase material (8-i) produced was further immersed into other 2-propanolwhile the laminated porous base material (8-i) was wet with the aboveimmersion solvent, and was then left to stand still at 25° C. for 5minutes. This produced a laminated porous base material (8-ii). Thelaminated porous base material (8-ii) produced was dried at 30° C. for 5minutes. This produced a laminated body (8). Table 2 shows the resultsof evaluation of the laminated body (8) produced.

Comparative Example 4

A porous film to which a coating solution had been applied as in Example2 was treated by a method similar to the method used in ComparativeExample 3. This produced a laminated body (9). Table 2 shows the resultsof evaluation of the laminated body (9) produced.

TABLE 2 α rate (%) Curl measurement Example 1 100 A Example 2 100 AExample 3 84 A Example 4 87 A Example 5 63 A Example 6 74 A Example 7 36B Comparative Example 3 21 C Comparative Example 4 27 C

[Results]

For the laminated bodies (1) to (7), which were produced in Examples 1to 7 and each of which included a porous layer containing a PVDF-basedresin that contained crystal form α in an amount (α rate) of not lessthan 36% with respect to the crystal form α and crystal form β combined,the measurement results show that curls were prevented. On the otherhand, for the laminated bodies (8) and (9), which were produced inComparative Examples 8 and 9 and for each of which the α rate was lessthan 36%, the measurement results show that clear curls occurred.

The above indicates that the laminated body of each of Examples 1 to 7which laminated body has an α rate of not less than 36% is not easilycurled.

The cycle characteristic of a nonaqueous electrolyte secondary batteryincluding a laminated body depends on the ΔWI value of that laminatedbody. The ΔWI value of such a laminated body in turn depends mainly onthe ΔWI value of the porous base material included therein. Thelaminated bodies produced in Examples 1 to 7 were each produced with useof the porous base material produced in one of Production Examples 1 and2. As shown in Table 1, a nonaqueous electrolyte secondary batteryincluding the porous base material produced in any of ProductionExamples 1 to 3 showed a good cycle characteristic. It follows that anonaqueous electrolyte secondary battery including the laminated bodyproduced in any of Examples 1 to 7 understandably each showed a goodcycle characteristic as well.

The results of Production Examples, Examples, and Comparative Examplesdescribed above show that the laminated bodies produced in Examples 1 to7 can each impart an excellent cycle characteristic to a nonaqueouselectrolyte secondary battery including the laminated body as aseparator and are not easily curled.

INDUSTRIAL APPLICABILITY

The present invention is widely applicable in industrial fieldsinvolving use of nonaqueous electrolyte secondary batteries such aspersonal computers, mobile telephones, and mobile information terminals.

REFERENCE SIGNS LIST

1 Resin

2 T-die

3 Roll

4 Distance between T-die and rolls

1. A laminated body, comprising: a porous base material containing apolyolefin-based resin as a main component; and a porous layer on atleast one surface of the porous base material, the porous layercontaining a polyvinylidene fluoride-based resin, the porous basematerial having a value of ΔWI of not more than 2.5, the ΔWI beingdefined in Formula (1) below,ΔWI=WI₁−WI₀   Formula (1) where WI represents a white index defined inAmerican Standard Test Methods E313, WI₀ represents a WI value of asurface of the porous base material which WI value is measured with useof a spectrocolorimeter before the porous base material is irradiatedwith ultraviolet light with an intensity of 255 W/m², and WI₁ representsa WI value of the surface of the porous base material which WI value ismeasured with use of the spectrocolorimeter after the porous basematerial is irradiated with ultraviolet light with an intensity of 255W/m² for 75 hours, the polyvinylidene fluoride-based resin containingcrystal form α in an amount of not less than 36 mol % with respect to100 mol % of a total amount of the crystal form α and crystal form βcontained in the polyvinylidene fluoride-based resin, where the amountof the crystal form α is calculated from an absorption intensity ataround 765 cm⁻¹ in an IR spectrum of the porous layer, and an amount ofthe crystal form β is calculated from an absorption intensity at around840 cm⁻¹ in the IR spectrum of the porous layer.
 2. The laminated bodyaccording to claim 1, wherein the polyvinylidene fluoride-based resincontains (i) a homopolymer of vinylidene fluoride and/or (ii) acopolymer of vinylidene fluoride and at least one monomer selected fromthe group consisting of hexafluoropropylene, tetrafluoroethylene,trifluoroethylene, trichloroethylene, and vinyl fluoride.
 3. Thelaminated body according to claim 1, wherein the polyvinylidenefluoride-based resin has a weight-average molecular weight of not lessthan 200,000 and not more than 3,000,000.
 4. The laminated bodyaccording to claim 1, wherein the porous layer contains a filler.
 5. Thelaminated body according to claim 4, wherein the filler has avolume-average particle size of not less than 0.01 μm and not more than10 μm.
 6. A nonaqueous electrolyte secondary battery member, comprising:a cathode; a laminated body according to claim 1; and an anode, thecathode, the laminated body, and the anode being arranged in this order.7. A nonaqueous electrolyte secondary battery, comprising as a separatora laminated body according to claim 1.